578 



EXPLORATION GEOPHYSICS 



ing the thickness and the width of the rock deposit. The resulting resistiv- 

 ity depth curves were compared with the drill logs as shown in Figures 

 358 and 359. It was found that where the silicified sandstone was present 

 the resistivity values were high, ranging from 400 to 700 ohm-feet. (Fig- 

 ure 358.) Where no rock was present, the resistivity was low, with values 

 of about 50 to 200 ohm-feet, characteristic of the soil, clay, sand and shale 

 of the area. 



Fixed depth resistivity traverses 

 were run between drill holes to check 

 the location of the edge of the rock 

 layer. As evidenced by Figure 360, 

 it was possible to find the termination 

 of the rock body quite accurately even 

 when it was covered by overburden. 

 The interpretation of the resistivity 

 data was made by visual inspection of 

 the depth curves and traverses. In 8 

 days of field work, with a four-man 

 crew, 13 resistivity depth curves and 

 7 lines of constant-depth traverse 

 were run, the latter covering about 

 1000 lineal feet. It was found that 4 

 resistivity depth curves to an electrode 

 separation of 50 feet could be con- 

 veniently measured in an 8-hour day, 

 allowing travel time to and from head- 

 quarters. One constant-depth traverse 

 250 feet long, with readings at 10-foot 

 intervals, required about the same 

 time as one depth curve. The 13 depth 

 stations and 7 traverse lines constitute 

 20 units of geophysical work. It is 

 estimated that the cost of one unit is 

 about $50, 



The tests demonstrated that, under 

 the typical conditions existing at the 

 Fremont Butte deposit, resistivity 

 depth curves were directly compar- 

 able to and as conclusive as drilling, in 

 determining the presence or absence 

 of silicified rock. The resistivity depth measurements provided quantitative 

 data on the depth and thickness of the rock layer. Such measurements 

 were also an index of its character, for in general high values of resistivity 

 indicate better quality rock. The constant-depth traverses clearly outlined 

 the edges of the rock body. 



A comparison may be made of the speed of resistivity exploration with 



..t 



>/' 



■^SOIL 



'>^prsiLicE( 



♦CLAY AND ASH 



^♦SILICEOUS S.S. 



50 



100 



SCALt OF FEET 



Fig. 360. — Section based on constant-depth 

 resistivity traverse and drilling. Presence of 

 siliceous rock shown by high resistivity val- 

 ues. (Courtesy of U. S. Department of In- 

 terior, Bureau of Reclamation.) 



